If there were flashes caused by demo charges, they would be unmistakable, and everyone would have heard very loud reports. If they were silent cutter charges, we'd see sparks flying all over. The give-away though, is it is after the building is falling, and well after the core had started collapsing. If you see any light flashes at that stage, it would be from windows breaking (angular reflection variation) or maybe from friction when the steel broke. There for sure was a pressure wave in that area of the building, indicating something was going on with the floor trusses.